I'm in the process of converting an 84 Schwinn World to use as a beater. It has a 90mm fork spacing, and a 126mm rear spacing. Can a 100mm spaced front wheel be respaced to 90mm? It looks as if most have 2 locknuts on each side. Is bending the fork out half a cm on each side plausable as well?

i think so. i helped build a conversion in a similar state, and it all worked out just fine. file the dropouts if you must.

Do it.

Measure the hub, but most likely you can't get 10mm of spacers off of it. I'd just spread the fork.

Usually pulling on forks you get a pretty even spread since you're applying equal force to both sides, but if you want to be really precise, clamp it or stand on the bars and pull one side until it's 95, and then pull the other out to 100. Also, you may want to use a vise or a wrench to set the dropouts in a little so they're parallel rather than flared, but that's turbo anal.

Often the 90mm forks have dropouts made for this funky old sub-9mm axle size, so you may have to file them. A round file is like $7, though, no biggie and useful for other stuff anyway. (You also need a flat file, of course.)